Richmond revealed

I was at one of Rob’s excellent rather useful seminars yesterday.  As it was about 3D printing, Rob asked me to say a little bit about my printer.  Having not prepared to do anything, I relied on some bits of video I found on my tablet.  Thinking about it, I realised that although this blog has quite a few posts about specific parts of the building process, there isn’t really a post just simply describing the printer.  So this is it.  I call it Richmond.

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It’s inspired by Johannn Rocholl’s Rostock printer, but the design is my own.  I wish I could blame someone else for the design flaws, but I can’t.  The basic design features three columns made of extruded aluminium section (bought).  Up and down each column runs a carriage (the pink bits), with spring-mounted ball bearings to make it run smoothly and without wobbling.   Rob printed these for me.  Here’s a close-up of one.

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The blue parts are modifications (printed by me) to add extra ball-bearings.  The original design simply had screw in the plastic which gradually wore loose.  At the right of the picture, you can see the fishing line which is attached to the carriage to drive it.  This loops around a motor-driven pulley at the bottom of the pillar and and idler pulley at the top, to make it move up and down.  The grey rods are carbon fibre, and join the carriage to the tool head.  The rods are in pairs, to keep them in a parallelogram shape – this means that the print head can move in three axes, but will never rotate.  The tool head looks like this from above: The translucent tube in the centre guides the plastic filament into the heated part.

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The print head looks like this if you happen to be underneath it (which I don’t recommend, because it heats up to 220 Celsius):

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The underneath view shows you the hot end – the part that melts the plastic and squirts it out through a tiny (0.3mm) hole.  The plastic filament is driven through the flexible tube to the hot end by an extruder, in which it is pressed against a rotating gear.  It takes a surprising amount of force to push the filament though the tube and the hot end.  Here’s the machinery that does it (with its operator):

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As usual, these components were printed for me by Rob, but they are not my design.  Rather, I downloaded the designs from thingiverse.  There is no sense in reinventing the wheel, especially when so much effort has gone into making it work well.  The plastic filament is taken from a reel, which sits close by on a home-printed stand:

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The filament drive is powered by a NEMA17 stepper motor.  Each carriage is also driven by one of these, mounted at the bottom of each column:

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Each motor has a printed pulley on it (printed and then machined with my printed lathe, in this case) which drives the Spectra non-stretch fishing line to move the carriage.  The filament is kept under tension by an adjusting screw on the carriage.  How much tension?  Until it goes ‘ping’ rather than ‘boing’ when you pluck it, that’s how much.  As an aside, it also makes an interesting Aeolian harp if you take it outside on a windy day.  Each stepper motor moves as finely as 3200 steps per revolution, so with the pulleys I have this means about 55 steps per millimetre of vertical movement.

Also mounted at the base of each pillar is an adjustable mount for the print platform (which is just a circle of glass from a local glass shop).  These are ugly and badly designed, but they do the job for the time being.  When I redesign the base, they will go.  Using three supports means that I can ensure the print surface is as close as possible to being perpendicular to the pillars.  This is important for ensuring good prints, particularly so for getting the first layer of plastic to stick to the base.  If the base is not level relative to the  and y movement of the print head, then it’s likely that the first layer will vary between being too thin (resulting in nothing but an impression in the masking tape) and too thick (resulting in a strand of plastic not actually stuck to the base).  Bear in mind that this means the print plate has to be level to within plus or minus a tenth of a millimetre over its diameter, and you’ll see why this calibration has been such an issue for me.

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The hardware is mounted on a box made out of plywood I had lying around, painted purple because I had a can of purple spray paint to hand.  On the front of the box is a basic controller interface – an LCD and a simple rotary/click controller. 

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The LCD provides status information about the printer, and a menu system to allow various parameters to be adjusted without using an attached PC.  It is possible to fit it with an SD card holder, which will allow printing completely independently, but I have no need for that.  The display and controller are driven by and provide input to an an Arduino Mega controller inside the main box.  The arduino is fitted with a Ramps 1.4 shield, which provides the stepper motor driver electronics and the higher power switching for the hot end.  The arduino runs Marlin firmware, which generates the stepper motor motions required to translate standard cartesian G-codes to carriage movements for the delta configuration.  The firmware also controls the extruder, monitors the temperature of the hot end, and drives the LCD.  The power supply driving all this is also in the purple box.  It’s a scrap supply from a PC, which provides handy 5V and 12V outputs, at high enough currents to drive the motors and the hot end.

Looking back, it doesn’t seem that complicated.  Makes me wonder why it took me so long to build.

Turn again, Whittington!

As a devoted reader of my blog, you will know that I’m engaged in a continuing quest to improve the precision of my 3D printer.  The most recent improvement was to replace all the pivots in my original design, which were simply composed of screws passing through holes in plastic, with proper miniature ball bearings.  This has had a huge benefit – there is now very little play in the movement of the print head.  Printed shapes are much more precise than they were.

The next issue to tackle is one I’ve been aware of for a while, but have not had the means to fix.  First, a recap: the whole print mechanism is driven by three stepper motors.  Each stepper motor has a pulley mounted on its output shaft, which drives a belt made of stretch-free fishing line.  Each belt goes the length of a vertical column, passes around an idler pulley at the top, and is attached to a carriage, so as the motor turns, the carriage moves up and down the column (a picture may not be worth a thousand words in this case, but if I had had one to hand it would certainly have saved me fifty or so).  The amount by which the carriage moves depends on the number of steps the output shaft of the motor rotates and the diameter of the output pulley.  Knowledge of these two things allows the printer firmware to be calibrated for each motor with the number of steps required per millimetre of carriage movement.  Because this figure is set in firmware, it doesn’t matter what each pulley diameter actually is – I can simply measure it and do the calculation.  However…

The problem I found on measuring the pulleys (kindly printed for me by Rob) is that they are not perfectly circular – the diameter varies by about half a millimetre around the circumference.  Add to that the fact that when a pulley is mounted on the motor shaft there is a little added eccentricity, and you end up with a drive pulley whose radius varies by about 5% as it rotates.  Over long movements, this is not a problem – the eccentricity averages out – but for small movements this results in unevenness.  It’s particularly obvious on the first print layer, which is often thin.  It manifests itself as patches of thin or thick deposition, and it just won’t do!

The cure is obvious: I need pulleys which, when mounted on the motor shaft, are round.  The standard method of making round things is with a lathe.  I would dearly love to have a proper modelmaker’s lathe, but they are expensive.  I can’t really justify spending £400 to make three plastic pulleys. I tried printing some more pulleys myself, but the accuracy of my prints was no better than Rob’s (not surprising, given that the pulleys weren’t round). 

I need a lathe.  I have a 3D printer.  You are ahead of me.  I had a search online to see if anyone had published designs for a 3D printed lathe, and the only one I could find was this one,  by a guy calling himself Sublime.  It’s a great piece of work, but it doesn’t look precise enough for my purpose, and I don’t need the three-jaw chuck.  For the time being, I only need to turn pulleys.  So I stole some ideas from Sublime, and designed my own lathe.  Here’s a video of it in operation:

I’m happy to say that it works.  It’s far from perfect, but the pulleys I have turned with it now have a variation in radius of 0.1mm or less, which makes for much better prints.  Now it’s on to the next improvement: automatic calibration of the print surface.  Watch this space.